1. Field of the Invention
This invention relates to ion implantation utilizing tandem accelerators. More generally, this invention relates to any process which uses negative ions in procedures where ions are used to bombard a target.
2. Description of the Prior Art
In ion implantation or other processes where a beam of particles is incident upon a workpiece, there are a number of methods for generation of the particle beam with appropriate choice of species, energy, current and spatial extent. One method that has been used to obtain these projectiles is tandem electrostatic acceleration. In a tendam accelerator, negative ions, produced and mass analyzed near ground potential, are acclerated towards a positive high voltage terminal. In the terminal, the ions pass through a dilute gas (or a thin foil) where collisions occur between the electrons of the fast ions and those of the gas molecules (or the atoms in the foil). During these collisions, electrons are stripped from the ions changing their polarity from negative to positive. The positive ions are now repelled from the terminal and accelerated a second time back to ground potential. After acceleration, the ions are magnetically analyzed a second time to select the appropriate charge state and these ions now constitute the particle beam, the generation of which was desired. In the case of ion implantation processes these ions enter an end station where they are directed at semiconductor wafers.
For efficient use of these accelerators, an intense source of negative ions in necessary. Previously, negative ions have been generated in a number of ways. In one method, these ions have been produced in sputter sources similar to the prototypical source of R. Middleton disclosed in Nuclear Instruments and Methods, Volume 214, page 139 (1983). In this source, positive cesium ions are generated by surface ionization on a heated tungsten filament and accelerated towards a target which is negatively biased by five to ten kilovolts with respect to the filament. When the Cs.sup.+ ions strike the target, a fraction of the target atoms are sputtered from the surface and a fraction of these will be negatively charged (the pressure of Cs lowers the work function of the surface and enhances the negative ion yield). The negative ions are accelerated away from the target, focused, mass analyzed, and injected into a tandem accelerator. However, the yield of negative ions for species usable in semiconductor applications is low (less than one hundred microamperes) and limits the applicability of these sources to ion implantation.
A second method of producing negative ions is by charge exchange. Using gas targets, this technique was used to generate negative ions for the first tandem accelerators. Later, B. L. Donally and G. Thoeming indicated that large (greater than one percent) charge exchange fractions could be produced with metal vapors as the electron donor targets, as disclosed in the Physical Review at Volume 159 page 87 (1967). This speculation was shown to be accurate in the experimental work of Heinemeier et al. and of D'yachkov et al. (see, e.g. Nuclear Instruments and Methods, Volume 148, pages 65 and 425 (1978); Zh. Tech. Fiz. Volume 43, page 1726 (1973); and Prib. Tekh. Eksp. Volume 5, page 27 (1975)). However, these measurements yielded results of less than one hundred microamperes for the negative ion beam, which is insufficient for production-type ion implantation systems.